Generally, the surgeon uses 36Fr or 40Fr bougies. The bougie serves as a measuring device in the form of a long and flexible tube. Its inserted through the mouth, esophagus and stomach directly to the pylorus (opening from the stomach into the duodenum or small intestine).
The surgeon uses the bougie to create a bulge which helps guide the stapler when dividing the stomach. Once the new sleeve is created, the surgeon then removes the bougie.
Bougies vary in sizes and the unit of measurement is called a French (Fr) where 1F = 0.333 mm (1/3 mm). To put this into perspective, a 40F bougie = 1/2 inch. In the United States, standard bougie sizes range from 32-50Fr.